1. Field of the Invention
The present invention relates to a still image encoder for producing still image data from moving image data and coding the produced still image data.
2. Description of the Related Art
Conventionally, a image transmission apparatus is used for a video conference system to hold a conference with persons at a long distance, or the like. Recently, this image transmission apparatus is also used for a remote monitoring system to monitor an accident, a disaster and the like at a long distance in addition to the above-mentioned video conference system.
In this image transmission apparatus, to make an original broad band of the moving image data less than a band of a communication circuit or the like, the moving image data is coded into data conforming to MPEG (Moving Picture Experts Group), and then transmitted. Thus, the moving image data decoded at a receiving side is rougher than the original moving data, for example, the decoded moving image includes noises.
For example, in the video conference system, when moving image data obtained by taking characters written in documents or a blackboard is transmitted by the conventional video transmission equipment, in a receiving side, a receiving device receive the moving image data, and make a result of the decoding process display a display device. Thus, a taken image of caracters written in documents or blackbord is displayed by the display device. In this case, there is a possibility that it is difficult to read the characters.
Further, in the above-mentioned remote monitoring system, when an accident, a disaster or the like occurs, a fine image in which an object or the like moves unsatisfactorily is required as a image to be displayed by the display device rather than a rough image in which an object or the like moves satisfactorily.
In that case, it is also considered to use a still image encoder for a video conference system, a remote monitoring system and the like. That still image encoder codes one-frame data extracted from frame data to be moving data into data conforming to a loss-less coding method in JPEG (Joint Photographic Coding Experts Group), data conforming to a lossy coding method in JPEG and having a data loss less than in MPEG or the like (hereinafter, data of these two types is called "JPEG data"), data conforming to ITU (International Telecommunication Union)-T Recommendation H.261 Annex-D or another data.
However, as to a moving image signal conforming to NTSC (National Television System Committee), one frame is interlaced with two fields. Thus, there is a difference between timings at which two fields to be one frame are taken. Thus, when frame data for one frame (data for two fields) is extracted as still image data, an image of an object which moves dynamically is blurred, therefore, there is a case that the image can not be recognized visually.
It is also considered that one field is extracted from two fields to be one frame and is coded. In this case, however, a image quality lowers all over the image.